1. Field of the Invention
The present invention relates to a photographing control apparatus and method.
2. Description of the Related Art
In recent years, there have been calls for a radiographic image photographing apparatus compatible with diverse photographic modes such as moving image photographing and energy subtraction photographing, rather than only still image photographing, with a single device configuration. A radiographic image photographing apparatus compatible with these diverse photographic modes is shown in Japanese Patent Laid-Open No. 2005-287773, for example.
Here, operations in a moving image photographing mode shown in Japanese Patent Laid-Open No. 2005-287773 will be described using FIGS. 7 and 8.
FIG. 7, showing a conventional example, is a timing chart showing exemplary timing for irradiating X-rays and for accumulating and reading out electric charge in a sensor in moving image photographing mode. In FIG. 7, timing is shown in the case where, in the frame period of a single frame, reading out of electric charge in photoelectric conversion devices of the sensor is performed once after X-ray irradiation and then once without X-ray irradiation being performed.
Note that the reading out once after X-ray irradiation and once without X-ray irradiation, as shown in FIG. 7, both involve the reading out of electric charge from all of the photoelectric conversion devices (all pixels) of the sensor. Consequently, in FIG. 7, these readout periods are the same length (Tr). The reading out after X-ray irradiation is for obtaining an X-ray image, while the reading out without X-ray irradiation is for performing offset correction.
In FIG. 7, the relation of the following equation (1) holds, where Tf10 is the frame period of a single frame of an X-ray image, Tr is the charge readout period, Tw11 is the charge accumulation period when X-ray irradiation is performed, and Tw12 is the charge accumulation period when X-ray irradiation is not performed. At this time, the frame period Tf10 of a single frame is equal to the X-ray irradiation cycle.Tf10=(Tw11+Tr)+(Tw12+Tr)  (1)
An offset correction value Vo10 can be calculated by the following equation (2), where Vx is the pixel value read out when X-ray irradiation is performed, Vf is the pixel value read out when X-ray irradiation is not performed, and Vo10 is the offset correction value.Vo10=Vx−Vf  (2)
Since the offset correction value is proportionate to the charge accumulation period, the following equation (3) desirably is satisfied to completely remove offset.Tw11=Tw12  (3)
In this case, the charge accumulation period Tw11 can be calculated by the following equation (4), when equation (3) is substituted into equation (1).Tw11=Tf10/2−Tr  (4)
Since the charge accumulation period Tw11 when X-ray irradiation is performed is greater than an X-ray irradiation period Tx, as shown in FIG. 7, the following equation (5) needs to be satisfied, where Tx is the X-ray irradiation period.Tw11>Tx  (5)
FIG. 8, showing a conventional example, is a timing chart showing exemplary timing for irradiating X-rays and for accumulating and reading out electric charge in a sensor in moving image photographing mode. In FIG. 8, timing is shown in the case where, in the frame period of a single frame, reading out of electric charge in photoelectric conversion devices of the sensor is performed once after X-ray irradiation and then three times without X-ray irradiation being performed.
In FIG. 8, Tf11 is the frame period of a single frame of an X-ray image, Tr is the charge readout period, Tw13 is the charge accumulation period when X-ray irradiation is performed, and Tw14 is the charge accumulation period when X-ray irradiation is not performed. At this time, to completely remove offset, the following equation (6) desirably is satisfied, with similar effect to equation (3).Tw13=Tw14  (6)
Therefore, the frame period Tf11 of a single frame is given by the following equation (7).Tf11=k·(Tw13+Tr)  (7)
Note that k is a positive integer showing the number of times charge accumulation and readout is repeated in the frame period of a single frame, with k=4 in the case of FIG. 8, and k=2 in the case of FIG. 7.
In FIG. 8, an offset correction value Vo11 can be calculated by the following equation (8), where Vx is the pixel value read out when X-ray irradiation is performed, Vf1, Vf2 and Vf3 are the pixel values read out when X-ray irradiation is not performed, and Vo11 is the offset correction value.Vo11=Vx−Vfn  (8)
Note that n is an integer between 1 and k−1.
The charge accumulation period Tw13 when X-ray irradiation is performed can be calculated by the following equation (9), based on equation (7).Tw13=Tf11/k−Tr  (9)
Since the charge accumulation period Tw13 when X-ray irradiation is performed is greater than an X-ray irradiation period Tx, as shown in FIG. 8, the following equation (10) needs to be satisfied, where Tx is the X-ray irradiation period.Tw13>Tx  (10)
Here, equation (4) is included in equation (9), since a comparison of equations (4) and (9) shows that equation (4) is equivalent to equation (9) with k=2 substituted therein.
While Japanese Patent Laid-Open No. 2005-287773 shows the case where low voltage X-rays and high voltage X-rays are irradiated once each as an energy subtraction photographing mode, energy subtraction photographing of a moving image is possible if this process is repeated. Here, the operations in energy subtraction photographing mode for a moving image in this case will be described using FIGS. 9 and 10.
FIG. 9, showing a conventional example, is a timing chart showing exemplary timing for irradiating X-rays and for accumulating and reading out electric charge in photoelectric conversion devices of a sensor in energy subtraction photographing mode for a moving image. In FIG. 9, timing is shown in the case where, in the frame period of a single frame, reading out is performed once after low voltage X-ray irradiation, once after high voltage X-ray irradiation, and once without X-ray irradiation being performed.
In FIG. 9, the following equation (11) holds, where Tf12 is the frame period of a single frame of an X-ray image, Txl is the low voltage X-ray irradiation period, Txh is the high voltage X-ray irradiation period, Tw15 is the charge accumulation period when low voltage X-ray irradiation is performed, Tw16 is the charge accumulation period when high voltage X-ray irradiation is performed, and Tw17 is the charge accumulation period when X-ray irradiation is not performed.Tf12=(Tw15+Tr)+(Tw16+Tr)+(Tw17+Tr)  (11)
Offset correction values can be calculated by the following equation (12), where Vxl is the pixel value read out when low voltage X-ray irradiation is performed, Vxh is the pixel value read out when high voltage X-ray irradiation is performed, Vf is the pixel value read out when X-ray irradiation is not performed, Vol is the offset correction value when low voltage X-ray irradiation is performed, and Voh is the offset correction value when high voltage X-ray irradiation is performed.Vol=Vxl−Vf, Voh=Vxh−Vf  (12)
To completely remove offset, the following equation (13) desirably is satisfied, with similar effect to equation (3).Tw15=Tw16=Tw17  (13)
The low voltage X-ray irradiation period Txl and the high voltage X-ray irradiation period Txh may differ, provided that equation (13) holds.
Consequently, the charge accumulation period Tw15 when low voltage X-ray irradiation is performed can be calculated by the following equation (14), when equation (13) is substituted into equation (11).Tw15=Tf12/3−Tr  (14)
Since the charge accumulation period Tw15 when low voltage X-ray irradiation is performed is greater than the low voltage X-ray irradiation period Txl and high voltage X-ray irradiation period Txh, the following equation (15) needs to be satisfied.Tw15>Txl, Tw15>Txh  (15)
FIG. 10, showing a conventional example, is a timing chart showing exemplary timing for irradiating X-rays and for accumulating and reading out electric charge in photoelectric conversion devices of a sensor in energy subtraction photographing mode for a moving image. In FIG. 10, timing is shown in the case where, in the frame period of a single frame, reading out is performed once each after low voltage X-ray irradiation and without X-ray irradiation being performed, and once each after high voltage X-ray irradiation and without X-ray irradiation being performed.
In FIG. 10, the following equation (16) holds, where Tf13 is the frame period of a single frame of an X-ray image, Txl is the low voltage X-ray irradiation period, Txh is the high voltage X-ray irradiation period, Tw18 is the charge accumulation period when low voltage X-ray irradiation is performed, Tw19 is the following charge accumulation period when X-ray irradiation is not performed, Tw20 is the charge accumulation period when high voltage X-ray irradiation is performed, and Tw21 is the following charge accumulation period when X-ray irradiation is not performed.Tf13=(Tw18+Tr)+(Tw19+Tr)+(Tw20+Tr)+(Tw21+Tr)  (16)
Offset correction values can be calculated by the following equation (17), where Vxl is the pixel value read out when low voltage X-ray irradiation is performed, Vf1 is the following pixel value read out when X-ray irradiation is not performed, Vxh is the pixel value read out when high voltage X-ray irradiation is performed, Vfh is the following pixel value read out when X-ray irradiation is not performed, Vol is the offset correction value when low voltage X-ray irradiation is performed, and Voh is the offset correction value when high voltage X-ray irradiation is performed.Vol=Vxl−Vfl, Voh=Vxh−Vfh  (17)
To completely remove offset, the following equations (18) and (19) desirably are satisfied, with similar effect to equation (3).Tw18=Tw19  (18)Tw20=Tw21  (19)
At this time, the low voltage X-ray irradiation period Txl and the high voltage X-ray irradiation period Txh may differ, provided that equations (18) and (19) hold.
Consequently, the following equation (20) is given, when equations (18) and (19) are substituted into equation (16).Tw18+Tw20=Tf13/2−2Tr  (20)
Since the charge accumulation periods when X-ray irradiation is performed are greater than the X-ray irradiation periods, the following equation (21) needs to be satisfied.Tw18>Txl, Tw20>Txh  (21)
Normally, the low voltage X-ray irradiation period Txl and high voltage X-ray irradiation period Txh are predetermined, often so that the following equations (22) and (23), for example, hold.Tw18=Txl+α  (22)Tw20=Txh+α  (23)
If the photographing frame rate is high (e.g., approx. 30 fps), α often takes a value of a few milliseconds.
When arranged using equations (20), (22) and (23), the charge accumulation period Tw18 when low voltage X-ray irradiation is performed is given by the following equation (24), and the charge accumulation period Tw20 when high voltage X-ray irradiation is performed is given by the following equation (25).Tw18=Tfl3/4−Tr+(Txl−Txh)/2  (24)Tw20=Tfl3/4−Tr+(Txh−Txl)/2  (25)
However, generally, with an X-ray photographing apparatus, photographing is performed at a reduced frame rate in order to lower the amount of radiation to which the subject (object) is exposed. There is a method that involves increasing the value of k in equation (7), for example, in the case where the frame rate is reduced, that is, in the case where photographing is performed with a longer frame period. Here, k=2 in FIG. 7, and k=4 in FIG. 8.
In the case of FIG. 8, the frame period Tf11 of a single frame shown in FIG. 8 will be twice that of the frame period Tf10 in FIG. 7, in order to satisfy the following equation (26).Tw11=Tw12=Tw13=Tw14  (26)
However, with the method that involves changing the value of k, the frame period Tf11 of a single frame can only be lengthened in integer multiples of the frame period Tf10 shown in FIG. 7. That is, it is difficult, in this case, to appropriately lower the frame rate in order to reduce the amount of radiation to which the subject (object) is exposed.
Another method in which photographing is performed with a longer frame period in order to reduce the amount of radiation to which the subject (object) is exposed involves lengthening the charge accumulation period. A timing chart thereof is shown in FIG. 11. FIG. 11, showing a conventional example, is a timing chart showing exemplary timing for irradiating X-rays and for accumulating and reading out electric charge in photoelectric devices of a sensor in moving image photographing mode. In FIG. 11, timing is shown in the case where k=2 and the charge accumulation period has been lengthened in comparison to the example in FIG. 7.
In FIG. 11, Tf14 is the frame period of a single frame of an X-ray image, Tr is the charge readout period, Tw31 is the charge accumulation period when X-ray irradiation is performed, and Tw32 is the charge accumulation period when X-ray irradiation is not performed.
Here, the relation of the following equation (27) holds if the frame period Tf14 of a single frame is greater than the frame period Tf10 in FIG. 7.Tw31>Tw11  (27)
However, the amount of dark current flowing to the photoelectric conversion devices of the sensor increases when the charge accumulation period is lengthened, as shown in FIG. 11, which increases the amount of electric charge that accumulates in the photoelectric conversion devices. The signal-to-noise (S/N) ratio deteriorates when the amount of accumulated charge increases, narrowing the dynamic range. The same problems similarly exist in relation to energy subtraction photographing mode and stereo photographing mode.